1. Field of the Invention
The present invention relates to a turbo chiller, a compressor therefor, and a control method therefor in which surging can be avoided.
2. Description of Related Art
In turbo compressors, such as centrifugal compressors, having a high head and a low volume flow, rotating stall and accompanying surging generally occur.
As a control method for this kind of centrifugal compressor, a method is known in which the relationship between the intake temperature of the centrifugal compressor and the minimum rotation speed that avoids surging is obtained in advance, and the rotation speed of the centrifugal compressor is controlled on the basis thereof (see, for example, Japanese Unexamined Patent Application, First Publication No. HEI 1-200095 (page 4 lower right to page 5 upper left, and FIGS. 1 and 2 therein)).
However, when the turbo compressor is used in a turbo chiller, the turbo compressor experiences a large influence on the operating point as a result of variations in the cooling capacity. This is demonstrated below in the case of chilled water output. For example, in the event of changes in the chilled water inlet temperature in a chilled water obtaining device provided in the evaporator of the turbo chiller, the cooling capacity changes so as to maintain the chilled water outlet temperature at a constant set value. When the cooling capacity changes, the condenser pressure and the condenser temperature at the high-pressure side also change accordingly. At this time, as in the known art described in the above-mentioned Japanese Unexamined Patent Application, First Publication No. HEI 1-200095, since the compressor rotation speed is controlled on the basis of the compressor inlet temperature alone, the operating point of the turbo compressor cannot be accurately determined, and thus operation in a stable region where rotating stall is avoided cannot be achieved.
Accordingly, by controlling the compressor rotation speed of the conventional turbo chiller, it is operated at a rotation speed at which rotating stall does not occur for the entire range of capacities at a prescribed compression ratio (“head”).
However, the rotation speed when operating at partial load becomes higher than the minimum rotation speed at which rotating stall can be avoided, thus resulting in wasteful energy consumption.